Multilayered ceramic circuit substrates contain patterned metal layers which act as electrical conductors sandwiched between ceramic layers which act as electrical insulators. These ceramic substrates are formed by laminating together thin green sheets of glass particles or an admixture of glass and crystalline particles mixed with binders and patterns of paste containing metal particles mixed with binders for forming conductors between, the ceramic greensheet. This green laminate is fired to burn off the binder materials, fired to coalesce the particles to a dense state further fired if necessary to crystallize the densified glass particles or to further crystallize the admixture of glass and crystalline particles to form an insulator and to coalesce the metal particles to form conducting metal lines.
Terms such as ceramic and glass ceramic are often used interchangeably in the art. To avoid confusion for the purpose of this application the following definitions will be used. The term ceramic has the following meaning: an aggregate of randomly oriented crystallines wherein the interstices between crystallines may contain uncrystallized material such as glass. The terms coalescence or densification refer to a heat treatment to reduce the density of pores in greensheets. The term crystallization refers to further heating after coalescence or densification or to heating if there is no coalescence or densification step to form crystallites from a glass. The term sintering refers to the heat treatment required to form the final ceramic. Sintering of a greensheet of an admixture of glass particles and crystalline particles is a heat treatment to coalesce or densify the greensheet plus a crystallizing heat treatment only if further crystallization is required. The term sintering temperature means, for a green sheet requiring crystallization, the crystallization temperature. The term sintering temperature means, for a greensheet not requiring crystallization, the coalescence temperature. Substrates made of ceramics requiring high temperatures for particle coalescence and densification, such as alumina, restrict the choice of co-sinterable conducting metallurgics to high melting point metals, for example refractory metals, such as molybdenum, tungsten, platinum, palladium or a combination of these with each other or certain other metals and precludes the use or preferable electrical conductors such as gold, silver and copper which have melting points less than the alumina sintering temperature. Alumina is a good insulator, has high thermal conductivity and has good strength. The dielectric constant of alumina is about 10.
Materials often referred to as glass-ceramics have been intensively studied in recent years for use as circuit substrates. These ceramics generally have low dielectric constant, a low thermal coefficient of expansion which is close in value to silicon and a low sintering temperature. The low sintering temperature permits the use of low melting point metals, such as copper and noble metals, for electrical conductors. Noble metals have low resistives comparable to copper. However, copper is less expensive and, therefore, its use substantially reduces manufacturing cost. When an oxidizable metal, such as copper, silver, molybdenum is used as the electrical conductor, it is necessary that thermoplastic organic binder materials contained within the green sheet used to form the ceramic and contained within the paste used to form the copper conductors be depolymerized and burned out in an atmosphere and at a temperature wherein the copper is not substantially oxidized.
Failure to burn out binders results in less than desirable ceramic properties. For example, if the binder is not fully burned out, residual carbon is left in the sintered ceramic which modifies the ceramic dielectric constant and inhibits complete densification. With only 0.1% residual carbon content the ceramic may be black having an apparent dielectric constant greater than 1000 and, rather than being an insulator, the fired ceramic will be a semiconductor. An oxidizing atmosphere is generally needed to burn out the binder.
Removal of the binder is complicated by the additional requirement that the burn-out ambient not excessively oxidize the oxidizable metal lines and planes. If the metal is excessively oxidized, the metal oxide diffuses into the ceramic and changes the dielectric properties of the ceramic. Also, when metal is excessively oxidized it expands causing stress within the green laminate which can result in delamination and cracking of the green laminate. Such cracks may not be removed by the sintering heat treatment thereby resulting in a ceramic weakened by cracks.
Quite surprisingly, applicants have discovered that by controlling the oxidation and reduction (dioxidation) of the metal lines this cracking can be avoided while providing substantial improvement in the interface region between the metal and ceramic as described herebelow.
Two references generally describing binder bum-out and the fabrication of ceramics are U.S. Pat. No. 4,234,367 to Herron et al. and U.S. Pat. No. 4,504,339 to Kamehara et al., the teachings of both of which are incorporated herein by reference.
Herron et al. U.S. Pat. No. 4,234,367 describes a method for forming sintered ceramic substrates containing multilevel, interconnected circuit patterns of copper-based conductor films obtained by heating the green laminate composed of particles of crystallizable glass in an ambient of hydrogen and water to a burn-out temperature of about 700.degree. C. to 800.degree. C. at a rate of 1.degree. C. to 3.degree. C. per minute. A lower burn-out temperature would take a prohibitively excessive amount of time for carbon removal. The binder burn-out time is about 11 hours as is apparent from FIG. 4 of the Herron et al. patent. This time is needed: 1) to avoid bloating of the ceramic article caused by entrapped volatile products which cause the article to expand instead of contracting on sintering, 2) to fully oxidize carbon in the binders, 3) to avoid drastic volume changes in the copper conductor resulting from the copper-oxide formation, and 4) to maintain reducing to neutral conditions for copper. After the binder is burned out the laminate is sintered in a neutral atmosphere to form the ceramic material by first heating to coalesce the laminate to dense state and thereafter heating to form crystals from the densified glass particles. During binder burn-out Herron et al. encountered difficulties alternating air and forming gas for purposes of oxidizing copper and reducing any formed copper oxide to copper without drastic volume changes resulting from the copper oxide formation. Applicants have discovered that by alternating environments after burn out interfacial region of controlled thickness can be formed. U.S. Pat. No. 4,504,399 to Kamehara et al. describes a method for forming sintered ceramic substrates containing multilevel, interconnected circuit patterns of copper-based conductor films from a green laminate composed of an admixture particles of glass and particles of crystallites. Binders are burned-out by firing the substrate in a controlled ambient of an inert atmosphere, e.g. nitrogen, containing water vapor at a partial pressure from 0.005 to 0.3 atmospheres at a temperature between 550.degree. C. and 650.degree. C. for a time sufficient to burn out the binders in the green laminate and thereafter raising the temperature in an inert atmosphere without water to the sintering temperature to coalesce the green laminate to form the ceramic. The binder burn-out time is about 8 hours as is apparent from FIG. 2 of the Kamehara et al. patent. The burn-out temperature upper bound is limited to 650.degree. C. because if the burn-out temperature used were higher, the glass components of the green sheets would begin to coalesce hindering further binder removal.
Both the patent to Kamehara et al. and the patent to Herron et al. teach burning out binders in an atmosphere sufficient to oxidize the binder but insufficient to oxidize the embedded copper conductors. It has been found that following methods such as these results in separation of the metal patterns from the ceramic material in the fully sintered ceramic structures. Also, after thermal cycling of the fully sintered structure Over the use temperatures of a semiconductor chip package substrate of about 0.degree. C. to about 125.degree. C., these existing separations can be exacerbated and new separations, not previously existing can occur. These separations can occur around metal lines within a metallization layer of the multilayer structure or around the metal vias which pass through the individual ceramic sheets to electrically connect conductive metal patterns on adjacent metallization layers. These separations arc undesirable since they structurally weaken the multilevel metal/ceramic structure, they weaken the adhesion of the metal patterns to the ceramic material, they provide regions within which residual chemicals used in processing can accumulate as a source of contamination and they permit the external atmosphere to penetrate the structure possibly causing corrosion therein.
It is an object of the present invention to provide a metal embedded within a fully sintered ceramic wherein the interface between the ceramic and metal is substantially free of voids.
A method of the present invention forms sintered ceramic substrates containing multilevel, interconnected circuit patterns of metal conductor films obtained by first firing in an inert atmosphere to drive out (or volatilize) most of the binder, thereafter firing on an oxidizing atmosphere, preferably an oxygen containing atmosphere, to burn out the residual binder, thereafter firing in a reducing atmosphere, preferably a hydrogen containing atmosphere. Firing in the oxidizing atmosphere and the reducing atmosphere can be repeated as many times as necessary.
U.S. Pat. No. 4,627,160 to Heron et al. is directed to adding catalyst to facilitate binder removal and states at Column 4, lines 7-16 that various efforts were made to completely remove the binder and that difficulties were encountered with various neutral or reducing atmospheres which include alternative air and forming gas to oxidize carbon or reduce copper oxide without drastic volume changes resulting from copper oxide formation. Applicants have discovered that by alternative oxidizing and reducing atmospheres and controlling the temperatures and length of time in these atmospheres these undesirable results can be substantially eliminated.
U.S. Pat. No. 4,050,956 to deBruin et al. describes bonding a refractory oxide ceramic to a metal by heating in air to a temperature below the melting temperatures of the metal and ceramic to cause for some metals a chemical reaction therebetween to form a bond therebetween which is not microscopically observable, the reaction zone being less than about 250 .ANG. and to form for other metals a metal/ceramic interface having a sharp discontinuity over which there is evidence of diffusion of metal into the ceramic. Furthermore, ceramic/metal/ceramic couples were prepared by sandwiching a metal foil between the flat faces of two ceramic discs to which a slight pressure was applied which the couple was held at an elevated temperature. In contradistinction, according to the present invention a microscopically observable interface region is fabricated between a metal and ceramic by heating alternatively in oxidizing and reducing atmospheres wherein the interface region contains microscopically observable, metal particles embedded in the ceramic.
EPO Published Patent Application No. 2 676 602, published May 18, 1988 describes burning out binders in a multilayered laminate by raising it from ambient to burn out temperature in an atmosphere by nitrogen gas containing oxygen. Residual carbon is burned out by maintaining the burn out temperature for several hours in an atmosphere of nitrogen gas containing hydrogen. In contradistinction, according to the present invention the oxidizing and reducing atmospheres are alternated for short period times to build up an interface region of metal particles embedded within the ceramic material.
U.S. Pat. No. 4,885,038 describes coating copper conductors with a polymeric coating to permit binder burn out in oxygen without oxidizing the copper conductors. Residual coating is removed after burn-out during sintering in a forming gas atmosphere.
U.S. Pat. No. 4,795,512 to Nakatani et al. describes forming a multi-layer ceramic having copper conductors by laminating PbO containing ceramic green sheets with pattern of copper oxide paste therebetween. The laminate is fired in an oxygen containing atmosphere to burn out binders. Since the copper forming past contains copper oxide there is no substantial further oxidation and consequently no substantial volumetric expansion as a result of oxidization. Thereafter, the laminate is fired at a temperature below the melting temperature of the ceramic, in an atmosphere of nitrogen containing hydrogen to reduce the copper oxide paste to copper but under conditions not sufficient to reduce the PbO in the green sheets. Thereafter, the laminate is fired in nitrogen to form a ceramic containing copper conductors. Since the paste contains copper oxide it appears that the paste under the heat treatments in reducing atmosphere coalesces into a copper conductor. There is no teaching of an interface region between the conductors and the ceramic. In contradistinction, according to the present invention, an interface region is fabricated by alternately treating a laminate to oxidizing and reducing atmospheres.
U.S. Pat. No. 4,863,683 to Nakatani et al. describes a process similar to that of U.S. Pat. No. 4,795,512 except that the ceramic is free of PbO. These are directed to burn out of binders in air which generally would cause complete oxidation of copper conductors which on sintering in a reducing environment would leave copper particles within the ceramic, thereby increasing its dielectric constant. To avoid or limit this oxidation zinc is added to the copper to avoid or substantially limit the oxidation of the conductors during burn out in air so that copper particle movement into the ceramic is eliminated or substantially limited. In contradistinction applicants have discovered that the generation of metal particles about a conductor in a ceramic can be beneficial if the metal particles are confined to a thin interface region. The interface region has a TCE intermediate of the ceramic and conductor TCEs which substantially avoids separation of the conductor and ceramic and cracking of the interface region during thermal cycling of the structure.
Firing in the oxidizing atmosphere both burns out the residual binder and oxidizes the surface of the metal patterns. Since there is a volumetric increase upon formation of the metal oxide, the oxidized surface of the metal grows outward into the surrounding ceramic. When the oxidized metal surface is exposed to the reducing atmosphere, the metal oxide is reduced leaving metal particles in the ceramic surrounding the metal pattern. The oxidation step must be controlled so as not to expand the oxidized metal pattern surface too much, since excess oxidation can form cracks in the green ceramic sheets which are not annealed out by the subsequent full sintering of the metal/ceramic structure. Prior to full sintering the ceramic is porous since it has not been fully densified. The pores provide passages for the oxidizing and reducing atmospheres to penetrate into the structure. In order to create a sufficiently thick intermediate region around the metal patterns and in order not to crack the green ceramic around the metal patterns, multiple oxidation/reduction cycles may be necessary. This depends on the type of materials for the conducting patterns and the ceramics. With each oxidation/reduction cycle more metal particles are disposed about the periphery of the metal pattern and the metal particles are disposed further away from the metal pattern. The oxidation/reduction cycles are controlled so as not to dispose metal particles throughout the ceramic since to do so will increase the dielectric constant between the adjacent metal lines, thereby increasing the capacitive coupling there between. In a final step the metal/ceramic structure is fully sintered and densified. The pores within the ceramic are closed, leaving a region around the metal patterns containing a mixture of metal particles within a fully sintered ceramic matrix which is substantially the same ceramic material as the surrounding ceramic material and integral therewith. The final sintering step is preferably done under pressure to push ceramic material into the pores of the intermediate region. The intermediate region or collar around the metal patterns has a thermal coefficient of expansion (TCE) which is intermediate between that of the metal and that of the ceramic. It has been found that of metals disposed within a ceramic without the intermediate region, the metal will separate from the ceramic because of the difference in TCE between the metal and the ceramic. The intermediate region avoids this problem.
It is an object of the present invention to provide an interface between an electrically conducting region and a ceramic region wherein the interface is capable of withstanding differential expansion between the metal and ceramic regions as a result of thermal cycling of the structure.
It is another object of the present invention to provide an interface region which is a combination of particles of an electrically conducting material embedded within a ceramic.
It is another object of this invention to provide an interface region wherein the electrically conducting particles are the same metal material as the electrically conducting region and wherein the ceramic in the interface region is the same ceramic as the ceramic region of the structure and wherein the ceramic in the interface region forms a unitary structure with the ceramic region.
It is another object of the present invention to fabricate a structure having an interface region by sintering an electrical conductor forming composition embedded within a green ceramic precursor, alternately in an oxidizing and a reducing atmosphere.
It is another object of the present invention to repeat the oxidizing and reducing atmosphere sintering treatment as many times as necessary to form the interface region.
These and other objects, features and advantages of the present invention will be readily apparent to those of skill in the art from the following detailed description and the drawings appended thereto.